One type of endoprosthesis device, commonly referred to as a stent, is placed or implanted within a blood vessel for treating stenoses, strictures, or aneurysms in the blood vessel. These devices are implanted within the vascular system to reinforce diseased, partially occluded, weakened or abnormally dilated sections of the blood vessel. Stents also have been successfully implanted in the urinary tract or the bile ducts to reinforce and prevent neoplastic growth in blood vessels. One common procedure for implanting the stent is to wrap the stent around a balloon catheter, to place the stent in the diseased portion of the vessel, for example, and then to inflate the balloon to secure the stent in place in the vessel.
Stents must be manufacturable and have suitable physical properties including suitable radiopacity, biocompatibility, and mechanical properties such as hoop strength, fatigue resistance, and corrosion resistance. Many stents today are composed of stainless steel or other metals which in their product thickness are not easily detectable using X-ray fluoroscopy. Stents have been constructed from tantalum wire, as disclosed in U.S. Pat. No. 5,135,536. However, tantalum wire is very radiopaque and thus, produces a very bright X-ray fluoroscopy image, which obscures the X-ray image resulting from the surrounding tissue and prevents the use of Quantitative Coronary Angiographic techniques for lumen size determination.
It is currently difficult to alter the radiopacity of an endoprosthesis without changing its physical properties. Physical properties of the endoprosthesis are very important. Some stents have very limited compliance characteristics, making them not particularly well suited for use in curved vessel pathways. For example, using stents having a generally rigid cylindrical shape in curved vessel pathways typically requires the stents to have a very short length and to be strung out along the curved pathway. Also, such stents are often delivered separately, thereby increasing the invasiveness of the procedure.
Previous approaches in endoprosthesis construction have used devices with good hoop strength. When stents are provided in relatively large vessels or deployed within a vessel susceptible to external forces, such as within the leg, good hoop strength is important to resist forces which tend to collapse the endoprosthesis.
Other endoprostheses exhibit less hoop strength but are more compliant and are better suited to conform to the contour of the vessel, rather than being so non-conforming as to misshape the vessel after deployment. A typical disadvantage of more compliant stent devices is that they tend to deform upon or after deployment and present stenting surfaces that can lack desirable uniformity throughout the working surface area of the stent. A non-uniform working surface area of the stent is especially evident during expansion of the stent from its collapsed, insertion diameter to its expanded, implanted diameter. At times, this lack of uniformity upon expansion is exacerbated by folds or other non-uniformities in the balloon on which the stent is mounted for deployment.